The determination of the activity of Creatine Kinase (ATP: creatine-phosphotransferase, E.C. 2.7.3.2; abbreviation; CK) in human serum is considered the most sensitive laboratory method for diagnosing diseases of skeletal muscle and myocardial infarction. However, differentiation between trauma of skeletal muscles and the myocardium is difficult, especially in making a differential diagnosis of myocardial infarction. Determination of total CK activity results in unreliable differentiation.
CK occurs in the body in the form of three isoenzymes; creatine kinase-MM in muscles; creatine kinase-BB in the brain; and hybrid creatine kinase-MB, consisting of an M and a B-subunit in the myocardium. CK activity in blood serum is normally due to the creating kinase-MM isoenzyme, because creating kinase-BB does not turn over rapidly and creatine kinase-MB is restricted to certain organs, for example, the myocardium. However, when the myocardium is damaged, as in cardiac infarction, creatine kinase-MB is released into the blood serum and can be detected there.
Quantitative determination of creatine kinase-MB and creatine kinase-MM in the serum is considered the most sensitive laboratory method and provides the greatest evidence in differential diagnosis of cardiac infarction. It is true that creatine kinase-MB is present in other organs, for example, the pancreas, the diaphragm, the aorta, the lungs and the uterus, as well as in the myocardium but the activity thereof in these organs is about 100 times less than in the myocardium, so that any creatine kinase-MB activity liberated from these other organs is below the limits of detection.
U.S. Pat. No. 4,067,775 discloses a method for determining the enzymatic activity of creatine kinase-MB is a biological sample. The method involves:
(a) incubating the creatine kinase-containing sample with polyclonal antibodies which inhibit the enzymatic activity of the M-subunit of creatine kinase (CK) isoenzymes MB and MM; and
(b) determining the enzymatic activity of the creatine kinase B-subunit after inhibition of the M-subunit.
The level of creatine kinase-MM normally in human blood is about 25 to 100 times greater than creatine kinase-MB and about 1,000 times greater than creatine kinase-BB. Any assay for creating kinase-MB must therefore be extremely sensitive. At least 99.5 percent of the creatine kinase-MM activity must be suppressed. Otherwise the activity of creatine kinase-MB will be completely masked. The polyclonal antibodies of the above prior art do not necessarily provide the desired level of creatine kinase-MM inhibition. Moreover, polyclonal antibodies suffer from lot-to-lot variation and the inhibition of creatine kinase-MB activity is so high (typically greater than 60 percent) that the sensitivity of the assay is reduced.